This invention relates generally to a process for synthesizing thionyl chloride and phosphorus oxychloride, and more particularly to a process capable of utilizing a fluctuating supply of sulfur dioxide to synthesize thionyl chloride and phosphorus oxychloride.
Sulfur dioxide is a by-product generated by many industrial processes, typically including processes requiring thionyl chloride. Most of this waste sulfur dioxide is condensed and disposed at considerable cost. As a result, it would be economically and environmentally advantageous to convert waste sulfur dioxide to usable products such as thionyl chloride and phosphorus oxychloride.
Thionyl chloride, SOCl.sub.2, and phosphorus oxychloride, POCl.sub.3, are obtainable from sulfur dioxide. A first synthetic route reacts sulfuryl chloride, SO.sub.2 Cl.sub.2 (readily formed from SO.sub.2 and Cl.sub.2) with phosphorus trichloride as described by Krumbiegel in German Patent 415,312: EQU PCl.sub.3 +SO.sub.2 Cl.sub.2 .fwdarw.POCl.sub.3 +SOCl.sub.2
A second synthetic route reacts chlorine, sulfur dioxide, and phosphorus trichloride as described by Schudel in U.S. Pat. Nos. 1,753,754 and 1,788,959: EQU Cl.sub.2 +SO.sub.2 +PCl.sub.3 .fwdarw.POCl.sub.3 +SOCl.sub.2
However, the SOCl.sub.2 and POCl.sub.3 obtained from either batch reaction is difficult to separate from unreacted PCl.sub.3 or SO.sub.2 Cl.sub.2 due to the relatively similar boiling points involved. The boiling point of sulfuryl chloride is 69.degree. C., phosphorus trichloride 76.degree. C., thionyl chloride 79.degree. C., and phosphorus oxychloride 107.degree. C. PCl.sub.3 or SO.sub.2 Cl.sub.2 contamination in the product mixture renders the recovery of high purity SOCl.sub.2 and POCl.sub.3 economically unfeasible.
A need therefore exists for a method of converting commercial by-product streams of sulfur dioxide to valuable thionyl chloride and phosphorus oxychloride products which are essentially free of unreacted reagents. The present invention addresses that need.